Radio amateurs are fortunate to enjoy exclusive access to a few frequency bands, meaning that they are not allocated to any other radio services. Other bands are shared with one or more other services on a co-primary basis. Access to some bands is on a secondary basis, meaning that amateur stations may not cause harmful interference to primary services and must accept any interference from them.
In the Table of Frequency Allocations a primary allocation is shown in upper-case letters (AMATEUR) while a secondary allocation is shown in lower case (Amateur). To understand the relative status of services in shared bands it is also important to read the relevant footnotes. A summary of the amateur spectrum allocations can be downloaded here.
Defending secondary allocations is a growing challenge, especially in the low microwave frequencies that are under severe pressure from commercial broadband services.